The present invention relates to an amplifier. It is particularly suitable for, but not limited to, use with a power amplifier in a portable radio telephone.
Portable radio telephones operate from an integral battery, and as such have only a finite energy store. Users of portable telephones are increasingly demanding longer operating times from their telephones between re-charging cycles.
Generally, the single largest consumer of energy in a portable telephone is the transmitter, and particularly the power amplifier (PA) stage of the transmitter. Not only does the PA have to operate at relatively higher power levels than the rest of the telephone, it must transmit the required signals while operating in a substantially linear mode of operation to ensure that transmissions conform to the defined standards applicable to the operating mode.
Generally, amplifiers operating in a linear mode are not particularly efficient, and so a compromise must be made between battery life and conformance to the specification. Since the specification must be complied with for a particular telephone to be usable with a particular system, the reduction in battery life necessitated by the amplifier's operating mode is generally accepted.
In prior art portable radio telephones, the transmission path can be notionally divided into several distinct sections. The information to be transmitted is first generated from input speech or data. The digital data created from this process is then modulated at low power to conform to the particular communication standard with which the telephone operates. The analogue Radio Frequency (RF) modulated signal is then boosted in power using a power amplifier, which applies gain to the signal so that it is suitable for transmitting over an air interface using an antenna.
As mentioned, a problem with prior art power amplifiers in portable radio telephones is that they tend to be relatively inefficient. The amplifiers tend to be operated in modes which result in relatively large amounts of the power input to the amplifier being wasted, primarily as heat. This results in shorter battery life and hence shorter talk times than could be otherwise achieved. Other inconveniences may be more frequent re-charging intervals and/or larger batteries.
Efficiency, η, is given by the following formula:
                    η        =                              RF            ⁢                                                  ⁢                          Power              out                                            Power                          i              ⁢                                                          ⁢              n                                                          (        1        )            
A typical figure for η in prior art GSM power amplifiers is in the region of 45%. Therefore, over half of the power input to the amplifier from the battery is wasted, and not converted into the transmitted RF signal.
Aside from the impact such wasted power has on battery life, the heat produced necessitates relatively bulky heat sinks around the PA circuitry to dissipate the unwanted heat. This leads to larger, heavier telephones, unpopular with users.
An ideal power amplifier would have an efficiency, η, of 100% i.e. all the power taken by the amplifier from the power supply would be converted into transmitted RF power, and none would be lost in the form of heat. Ideal amplifiers are of course not possible, but improved PA efficiency is highly desirable.
A known technique which can be used to produce more efficient amplifiers is known as Envelope Elimination and Restoration (EER). This was first disclosed in a paper by L Kahn—“Single Sided Transmission by Envelope Elimination and Restoration”—Proc. IRE, July 1952, pp. 803-806.
The technique is also known as polar modulation, and is described below with reference to FIG. 1 of the accompanying drawings. A low power modulated signal 200 is input to the system 100 at point 110. The modulated signal 200 is both phase modulated (PM) and amplitude modulated (AM). The signal is split using a splitter 120 into two equal components. The first component is applied to detector 130 which outputs a signal corresponding to the envelope 210 of the signal 200. Signal 210 therefore corresponds to the AM information in the signal 200.
The second signal is applied to limiter 140 which outputs a hard limited version 220 of the signal 200. Signal 220 has all the amplitude variation removed from it, and therefore corresponds to the PM information in the signal 200.
The detected signal 210 is applied to amplifier 150, which is used to drive the power supply to the power amplifier 160. In this way, the power amplifier 160 can operate with a constant gain, and the variation in the envelope of the input signal 200 can be recreated by altering the power supply in sympathy with the envelope of input signal 200, as represented by signal 210.
The resultant output signal 230 corresponds to an amplified version of the input signal 200, and is ready for transmission by an antenna (not shown).
To describe the situation mathematically, consider an input signal 200 given by f(t).
The amplitude of f(t) is given by g(t), and the phase of f(t) is given by h(t). Using standard complex notation:f(t)=g(t)·ejh(t)  (2)
It can be seen from FIG. 1 that g(t) is represented by signal 210, and that h(t) is represented by signal 220.
The output signal 230 is given by F(t), and is an amplified version of f(t). The amplification is given by G.F(t)=G·f(t)  (3)
The technique used in the system of FIG. 1 has been known for nearly 40 years, but applying it to devices used in the field of portable telecommunications has proved problematic, primarily due to the frequency of operation of such devices.